1. Field of the Invention
The present invention relates to an optical scanning device that irradiates an optical beam emitted from a light source and reflected by a deflector to a photosensitive element, to write electrostatic latent images, and relates to an image forming apparatus using the optical scanning device, an optical-scanning correcting method, and an image forming method.
2. Description of the Related Art
In a tandem type image forming apparatus that forms images of respective colors by one polygon motor simultaneously, positions and angles of optical elements slightly change due to heat generated by the polygon motor in the optical scanning device as an optical writing unit and due to environmental changes in the machine, thereby changing the scanning position of the optical beams with respect to the photosensitive elements. As a result, registration between colors, inclination of scanning lines between colors, and curvature of scanning lines between colors occur. These factors cause color misalignment of a color image to be synthesized. This phenomenon of the color misalignment is more particular in a sub scanning direction.
Accordingly, a method of providing a pattern image (a registration mark image) for detecting a misalignment amount in the sub scanning direction on a photosensitive drum or a transfer medium has been widely adopted. Thereby, the amount of color misalignment can be reduced based on the misalignment amount detected by a sensor from a pattern image transferred onto the transfer medium, for example.
According to this method, however, there is a problem that the pattern image is contaminated due to dust and dirt, since the misalignment pattern image is arranged near the photosensitive drum or the transfer medium (an intermediate transfer belt). Furthermore, when the photosensitive drum or the transfer medium is stained or foreign matter adheres thereon, the pattern image may not be written accurately. Detection may not be possible as a result, and even if detection can be made, the correction result may not be appropriate.
Accordingly, as means for solving this problem, there has been proposed a technique in which a sensor for detecting scanning positions of optical beams of respective colors is installed to detect fluctuations of mutual positions of respective beams, and the result thereof is reflected to the control of modulation timing of the optical beams, to correct color misalignment (for example, see Japanese Patent No. 3087748, Japanese Patent Application Laid-open Nos. 2000-235290 and 2004-287380).
However, in the technique for correcting color misalignment, since the optical beams reaching the sensor do not pass through an optical element to be passed at the time of writing an actual image, or pass through an optical element, through which the optical beams reaching a surface to be exposed do not pass (one for folding an optical path or for changing an imaging position), registration, which is considered to have been appropriately corrected based on the detection result of the sensor, may not be linked to an actual image.